I bought the heads about ten years ago when I was 16. I failed to realize they were different and now they just sit around collecting dust. The casting numbers are 3919840 and 3964291. They are in great shape and was wondering if the differences in cc's (104.9 to 106.9) is because the exhaust valve is larger on the 291 head(1.88 vs 1.72)? These heads I believe are closed chambered rectangle port. Would it be possible to enlarge the exhaust valve on the 840 head and have the cc's equal out? Thanks for your input.

Sure, the best way to do it is to actually cc all three areas and then port and such until they are the same cc for each.

The chambers and matching valve sizes, the intake ports and last, the exhaust ports!! This way they will match...

But it may be easier just to find one matching head for either numbered head... pdq67